Η δαχτυλήθρα είναι στο δάχτυλο.

Because Greek articles change to match the noun’s gender, number, and case.

• η = feminine singular nominative
• το = neuter singular nominative/accusative

So:

• η δαχτυλήθρα = the thimble
  because δαχτυλήθρα is a feminine noun
• το δάχτυλο = the finger
  because δάχτυλο is a neuter noun

English just uses the for everything, but Greek has different forms.

στο is a contraction of σε + το.

• σε = a preposition that can mean in, on, at, or sometimes to
• το = the for a neuter noun

So:

• σε το δάχτυλο → στο δάχτυλο

In this sentence, στο δάχτυλο means on the finger. Greek uses σε much more broadly than English uses in or on, so the best English translation depends on context.

After σε, Greek uses the accusative case.

But for many neuter singular nouns, the nominative and accusative forms are identical. That is exactly what happens here:

• nominative: το δάχτυλο
• accusative: το δάχτυλο

So the case does change grammatically, but the form happens to look the same.

There are two main clues:

• η δαχτυλήθρα is in the nominative, which is the normal case for the subject
• στο δάχτυλο is a prepositional phrase, so it gives location, not the subject

So the structure is:

• Η δαχτυλήθρα = subject
• είναι = verb
• στο δάχτυλο = location phrase

In other words: The thimble is on the finger.

Not always. είναι is the 3rd-person form of είμαι and it is used for both:

• he/she/it is
• they are

So by itself, είναι can mean either is or are.
In this sentence it means is, because the subject η δαχτυλήθρα is singular.

A rough pronunciation is:

• Η = ee
• δαχτυλήθρα = tha-khtee-LEE-thra
• είναι = EE-ne
• στο = sto
• δάχτυλο = THA-khtee-lo

A more accurate IPA version is:

• Η /i/
• δαχτυλήθρα /ðaxtiˈliθra/
• είναι /ˈine/
• στο /sto/
• δάχτυλο /ˈðaxtilo/

A few important sound notes:

• δ sounds like th in this
• θ sounds like th in think
• χ does not sound like English ch in chair
  It is a rough throat sound, like German Bach or Scottish loch

The accent marks show which syllable is stressed.

So here:

• δαχτυλήθρα
• είναι
• δάχτυλο

This matters a lot in Greek pronunciation. If you stress the wrong syllable, the word may sound unnatural or even become confusing.

The given order is the most neutral and straightforward:

• Η δαχτυλήθρα είναι στο δάχτυλο.

Greek word order is more flexible than English word order, so other orders are possible for emphasis or style. For example, a speaker could move στο δάχτυλο earlier if they wanted to highlight the location.

But for a basic learner, this sentence is a very natural standard pattern:

• subject + verb + place phrase

Yes. They clearly share the stem δαχτυλ-, which is connected with finger.

That can help you remember them:

• δάχτυλο = finger
• δαχτυλήθρα = thimble

So the words are not just similar by accident; they are historically related.

In a normal sentence like this, using the articles is natural and expected:

• Η δαχτυλήθρα
• στο δάχτυλο

Greek uses the definite article very often, sometimes more often than English does. If you removed the articles, the sentence would sound incomplete or unnatural in ordinary usage.

So for standard Greek, this full form is the one you should learn.